The present invention relates to a method of forming a film by spirally dropping liquid on an in-process substrate.
The spin coating method has been conventionally used for a lithography process. This method discards most of liquid dropped on a substrate outside it and only uses a few percent of the remaining liquid for film formation. Much solution to be used is wasted and discarded, causing environmentally unfavorable effects. When the method is applied to a square substrate or a large circular substrate 12 inches or more in diameter, turbulence occurs at an outer periphery of the substrate, causing the film thickness to be partially uneven.
There are disclosed several methods of evenly applying solution on the entire surface of a substrate. Jpn. Pat. Appln. KOKAI Publication No. 2-220428 describes the technique of providing a uniform film by dropping resist from many nozzles arranged in a row and spraying a gas or liquid onto a film formation face from the rear. In Jpn. Pat. Appln. KOKAI Publication No. 6-151295, a rod has many spray nozzles from which resist is dropped on a substrate to provide a uniform film. Jpn. Pat. Appln. KOKAI Publication No. 7-321001 describes the technique of using a spray head having many spray holes for coating by moving the spray head relative to a substrate. All of these coating apparatuses aim at providing a uniform film by arranging a plurality of dropping or spray nozzles in a row and scanning them along a substrate surface.
In addition to the above-mentioned coating method using an apparatus having a plurality of nozzles, there is provided a technique of forming a solution film by using a single liquid discharge nozzle and scanning it above an in-process substrate. This technique causes the problem that a single substrate requires a long process time or a large amount of solution to be used depending on how a nozzle is operated.
For solving these problems, Jpn. Pat. Appln. KOKAI Publication No. 2000-77326 discloses the coating technique by spirally supplying solution. This publication contains the description that, as coating conditions, it is desirable to perform coating by rotating a wafer at a low speed (say, 20 to 30 rpm) and moving a nozzle unit along a diameter direction (say, an X-direction) of this wafer. Another description is that it is important to keep a relative speed between a wafer and a nozzle unit constant. Namely, it is described that a nozzle's linear speed should be constant.
When the nozzle unit is moved at a constant speed, it is required to increase a rotational frequency inside an outer periphery of the nozzle for keeping the linear speed constant. When a 200 mm wafer is used, for example, a rotational frequency is assumed to be 30 rpm for a 100 mm radius. The rotational frequency is proportional to the reciprocal of the radius. The wafer needs to be rotated at 3,000 rpm for a radius of 1 mm or less. When the wafer is rotated at 3,000 rpm, the solution is instantaneously ejected outside the substrate even if the liquid coating starts from the center.
When the wafer is rotated at a constant low rotational frequency, a nozzle movement speed is very high at the center of the substrate. Even if a vibration is applied after coating to move the liquid, it is moved incompletely causing an uncoated region at the center, making it impossible to form a uniform film. There is the problem that discharging the solution at a constant linear speed disables formation of a solution film.